Combi is short for combination.
These HVAC systems were traditionally called a combination boiler since they were originally designed to combine all parts of a traditional central heating system into a single compact unit.
A combi boiler is a type of boiler that can provide both central heating and hot water. Modern combination boilers utilize condensing technology to efficiently heat your HVAC system water by extracting latent heat from the flue gases. When you require hot water or heating, a gas burner in the combination boiler is ignited. The heat from the flame is then transferred through the main heat exchanger to your primary circuit, at which point the heat is transferred directly to either the radiators or to your hot water taps through a plate heat exchanger. Many modern combination boilers also have a pre-heat system that allows your combination boiler to store a small amount of water that can be drawn from your boiler. The water is collected inside your boiler and maintained at your desired temperature level throughout the day, ready to be moved to your hot water tap as quickly as possible. Combination boilers are a common choice for many homeowners because of the several benefits they come with. These include but are not limited to their compact size, efficiency, money-saving ability, and their ability to provide hot water on demand. Combi boilers are designed to last anywhere between 10 to 15 years, depending on how regularly they are serviced and their model. With proper regular servicing, the lifespan of a combination boiler can be extended considerably.